Pulling rolls are used in the manufacture of sheet glass to apply tension to the ribbon of glass from which the sheets are formed and thus control the nominal sheet thickness. For example, in the overflow downdraw fusion process (see Dockerty, U.S. Pat. Nos. 3,338,696 and 3,682,609), pulling rolls are placed downstream of the tip or root of the fusion pipe and are used to adjust the rate at which the formed ribbon of glass leaves the pipe and thus determine the nominal thickness of the finished sheet.
A successful pulling roll needs to meet a number of conflicting criteria. First, the roll needs to be able to withstand the high temperatures associated with newly formed glass for substantial periods of time. The longer a roll can last in such an environment the better, since roll replacement reduces the amount of finished glass a given machine can produce and thus increases the ultimate cost of the glass.
Second, the roll must be able to produce sufficient pulling force to control glass thickness. In order not to damage the central portion of the ribbon that becomes the usable finished glass, the roll can only contact the ribbon over a limited area at its edges. Thus, the required pulling forces must be generated using only this area. However, the forces applied to the glass cannot be too large since this can create surface damage which can propagate into the usable central portion of the ribbon. Accordingly, the roll must achieve a balance between applying too little and too much force to the edge regions of the glass.
Third, the millboard material used in the construction of pulling rolls must be hard enough to resist process damage due to broken glass during production for extended periods of time.
Fourth, the pulling roll must not give off excessive amounts of particles, which can adhere to the glass and form surface defects known as onclusions. For glass that is to be used in demanding applications, such as substrates for flat panel displays, onclusions must be kept to very low levels since each onclusion will typically represent a defective region of the finished product (e.g., one or more defective pixels). Because of the hot environment in which pulling rolls operate, providing materials that can apply sufficient pulling forces to a glass ribbon and yet not give off particles when hot is a difficult challenge.
Pulling rolls are preferably designed to contact the glass ribbon at its outer edges, specifically, in regions just inboard of the thickened beads that exist at the very edges of the ribbon. A preferred construction for such rolls employs discs of a heat resistant material, such as millboard, which are mounted on a driven shaft. Examples of this construction can be found in Moore, U.S. Pat. No. 3,334,010, Asaumi et al., U.S. Pat. No. 4,533,581, and Hart et al., U.S. Pat. No. 5,989,170, which are incorporated by reference in their entirety and for the specific purpose of describing examples of construction for pulling rolls.
Millboard materials have been used commercially for many years as thermal insulation in gaskets, linings for fire-safe cabinets, and in the glass making industry as float roll covering materials. Early millboard compositions, such as those described in U.S. Pat. Nos. 1,594,417, 1,678,345, and 3,334,010, often contained cement binders and asbestos fibers to strengthen the resulting millboard and provide heat resistance in high-temperature applications. Health concerns related to the use of asbestos led to the development of asbestos-free millboard materials. U.S. Pat. No. 4,244,781, for example, discloses a millboard composition containing ceramic and organic fibers, pyrophyllite, and an inorganic binder. Similarly, U.S. Pat. No. 4,308,070 discloses a millboard containing a combination of cellulose fiber, barium sulphate, cement, and inorganic fiber.
Millboards comprised of washed ceramic fiber and incorporating various fillers and functional components have also been used as roll coverings for float line rolls in the manufacture of glass. These washed ceramic materials frequently contain approximately twenty or more percent of unfiberized material, or shot, of a size less than 100 mesh (0.0059 inches). This unfiberized material can cause microscopic defects in the glass sheet as it passes over the float line rolls. Once the binder is removed, these millboard materials can also become dusty and potentially create onclusions on the glass sheets.
Existing pulling rolls have not been able to fully satisfy the competing criteria of long high temperature life, controlled force application, hardness, and low contamination. Thus, there is a need in the art to obtain a pulling roll that achieves higher levels of such performance than existing pulling rolls.